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This paper reviews the problems encountered in eleven studies of Sr isotope analysis using laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) in the period 1995-2006. This technique has been shown to have great potential, but the accuracy and precision are limited by: (1) large instrumental mass discrimination, (2) laser-induced isotopic and elemental fractionations and (3) molecular interferences. The most important isobaric interferences are Kr and Rb, whereas Ca dimer/argides and doubly charged rare earth elements (REE) are limited to sample materials which contain substantial amounts of these elements. With modern laser (193 nm) and MC-ICPMS equipment, minerals with >500 ppm Sr content can be analysed with a precision of better than 100 ppm and a spatial resolution (spot size) of approximately 100 μm.

New trace element and Hf^Nd isotope data on post-glacial basalts from Iceland' s main rift zones are used in conjunction with literature data to evaluate the relative importance of source heterogeneity and the melting process for the final melt composition. Correlations between Hf and Nd isotope compositions and trace element ratios indicate that at least two source components are sampled systematically as a function of the degree and pressure of melting beneath Iceland. Strong depletion in Rb, Ba, U and Th and enrichment in Nb and Ta compared with La in the most enriched samples from the Reykjanes Peninsula and Western Rift Zone suggests that the enriched source component is similar to ancient recycled enriched mid-ocean ridge basalt (E-MORB) crust. Highly incompatible trace element ratios such as Nb/La and Nb/U and Pb isotope ratios are variable across Iceland. This observation suggests that either the enriched component is intrinsically heterogeneous, or that there is a larger proportion of the enriched source component beneath the Southwestern Rift Zone compared with the Northern Rift Zone. The relative effect of source heterogeneity and melting on the final melt composition was evaluated with a one-dimensional polybaric melt mixing model in which accumulated melts from a depleted MORB mantle and a recycled E-MORB crust are mixed in different ways. Two styles of melt mixing were simulated: (1) complete mixing of melts with variable proportions of the depleted mantle and recycled E-MORB components; (2) incomplete mixing with a fixed initial proportion of the two source components. Calculated pressure-dependent compositional changes using these simple two-component melting models can explain the observed trends in trace element ratio and isotope ratio diagrams for Icelandic basalts, even in cases where conventional binary mixing models would require more than two source components. The example of Iceland demonstrates that melt mixing during extraction from the mantle is a key process for controlling the trace element and isotope variability observed in basaltic lavas and must be evaluated before inferring the presence of multiple source components.

We have investigated the use of current amplifiers equipped with 10 12 ohm feedback resistors in thermal ionisation mass spectrometry (TIMS) analyses of sub-nanogram sample aliquots for Nd and Sr isotope ratios. The results of analyses using the 10 12 ohm resistors were compared to those obtained with the conventional 10 11 ohm amplifiers. We adopted a method where long baselines were measured before and after data acquisition to save time during sample analysis. The noise level on the new design 10 12 ohm resistors is 3-5 fold lower compared to the 10 11 ohm resistors, which results in a two fold better analytical precision on 143 Nd/ 144 Nd and 87 Sr/ 86 Sr analyses for beam intensities of 10 mV for the 143 Nd and 87 Sr masses, respectively. Reproducibility of 87 Sr/ 86 Sr on repeat analyses of 1 ng Sr sample loads is better using 10 12 ohm resistors compared to 10 11 ohm resistors and is a factor of 2 better compared to previous studies. The external reproducibility of 143 Nd/ 144 Nd for 1 ng Nd sample loads is a factor of 1.4 better compared to previously published Nd + results and is similar to those obtained using the more timeconsuming NdO + techniques. Reproducibility tests on 100 pg Nd and Sr samples of reference materials yield 176 ppm of 143 Nd/ 144 Nd and 92 ppm of 87 Sr/ 86 Sr. These data suggest that we can resolve variability in the fourth decimal place of Nd and Sr isotope ratios in geological samples as small as 100 pg.

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